Preparation And Properties Of Carbon Nanotube Gas/Humidity Sensors

Since the appearance of nerve agents, they are frequently used in war and terroristattacks. Therefore, the manufacture and development of nerve agent sensors are of greatsignificance for national and public security. Because of the high aspect ratio andspecific surface area, as well as the strong gas adsorption capacity, carbon nanotubes(CNTs) are ideal sensitive materals in gas sensors. In this dissertation, CNT thin filmswere prepared by airbrush, screen printing, chemical vapor deposition (CVD),electrophoresis and layer-by-layer self-assembly techniques, which were characterizedand analyzed with different methods. The planar interdigital capacitor and platecapacitor were fabricated, and the dimethyl methylphosphonate (DMMP) gas-sensingproperties of these CNT sensors were carried out and studied. The main research resultswere as follows:1. Single-walled carbon nanotubes (SWNTs) were deposited on interdigitalelectrodes (IDTs) by airbrush method, and it formed interdigital capacitive sensor. Thecapacitance of this sensor decreased rapidly when exposed to DMMP. The capacitanceresponse increased with the increase of DMMP concentration and was12.43%to12mg/m3DMMP. The sensor had a good reproducibility and selectivity. Meanwhile, theresistance response of airbrush SWNT sensor was also studied. The resistance of thissensor increased immediately when exposed to DMMP and the response increased withthe increase of DMMP concentration, but the resistance response was smaller than thecapacitance response by one order in magnitude. The capacitance responses of airbrushmulti-walled carbon nanotube (MWNT) sensor and SWNT/DKAP (poly{methyl[3-(2-hydroxyl,3,5-bistrifluoromethyl)phenyl]propylsiloxane}) composite film sensor weresimilar to the response of airbrush SWNT sensor, but their responses were sigmificatelysmaller and were1.39%and0.54%to12mg/m3DMMP, respectively. The DKAPtwined and wrapped SWNTs in the composite film, reducing the effective surface areaof CNTs and causing the capacitance response significately smaller than thecorresponding SWNT sensor. It is also found that when the airbrush CNT sensor withlarger initial capacitance and loss tangent, the CNT film was denser, the average pore size was smaller and the effective surface area for gas adsorption was larger, leading tohigher capacitance response.2. SWNT sensor was prepared on SiO2/Si substrate by screen printing, and theresistance response of this sensor to50ppm (250mg/m3) DMMP was16.4×10-3.Because there was residual ethyl cellu, etc. in the screen printed SWNT film, theresistance response was far less than the corresponding airbrush SWNT sensor. Thesensor had a very good reproducibility and long-term stability, but its resistancerecovered very slowly, which was more obvious at high concentration of DMMP. Thesensor had a good selectivity to the gases which were present in the air, such as water,oxygen and carbon dioxide. But the selectivity to the gases with redox, such as NH3,etc., and organic vapors, such as alcohols and aromatics, was very poor due to thestrong adsorption between these gases and the residual ethyl cellu. The capacitor formedby the two plates of the screen printed SWNT sensor had response to DMMP. Whenexposed to20mg/m3DMMP, the capacitance response was only0.028%, which wassmaller than the resistance response.3. SWNT gas sensors were fabricated on quartz crystal microbalance (QCM) bylayer-by-layer self-assmbly of polyelectrolyte poly(diallyldimethylammonium chloride)(PDDA) and SWNTs. It was found that the sensors had a significantly greater responseto humidity than to DMMP. The effect of raw SWNT materials with and withoutcarboxyl (-COOH) on the humidity sensing properties of these SWNT composite thinfilm sensors was studied and compared. The results showed that when the relativehumidity was changed from20.9%to80.2%, the relative frequency shifts of thePDDA/SWNT-COOH QCM sensor and PDDA/SWNT QCM sensor were16.64%and13.84%, respectively. The former had a sensitivity enhanced by20.03%, a betterselectivity and a shorter response time, but the recovery time was longer and thehysteresis was increased from3.9%to8.0%. It was attributed to that the relative contentof π-π*transition structures and the corresponding functional groups in modifiedSWNTs-COOH was much higher than that in modified SWNTs. However, thesensitivities of the two sensors to60%saturated vapor pressure concentration of DMMPwere only2.06%-2.17%.